1. Field of the Invention
This invention relates to a hydraulic device for supplying a hydraulic driving unit in an injection molding machine for processing synthetic materials comprising at least one variable capacity pump, a main line extending from the variable capacity pump to the driving unit, at least one control valve arranged in the main line and adapted to determine a quantity and a pressure, a control conduit which connects a controlling mechanism of the variable capacity pump with the main line downstream of the control valve, as well as at least one pressure transducing means arranged for transducing an actual pressure value downstream of the control valve, comparing means for comparing a nominal pressure value and supplying a first correcting variable for the controlling mechanism in dependency of result of the comparing means.
2. Description of the Prior Art
A hydraulic device of this kind for a hydraulic circuit of an injection molding machine for processing synthetic materials is known from German Patent 31 19 095. In this hydraulic device the volume flow requirement is adapted in an energetically favourable way to the consumer by a pressure-current regulated variable displacement pump and a control valve which is applied as adjustable ring spinner. To this end a control conduit measures the pressure downstream of the control valve, which leads the volume flow to the consumer and supplies this value as input quantity to the controlling mechanism. This input quantity is increased by a preset operational pressure gradient, so that the pressure drop is maintained constant via the control valve. The pump is run with maximum power during the whole operating time, so that even in those cycle parts in which no large volume quantities have to be delivered, a large leakage occurs in the pump region. Moreover the maximum speed of the pump has to be in derivative action over the complete operating time.
According to German Published Application 39 19 823 the hydraulic consumers (21,22) in a hydraulic device are connected with pumps (311,321), which are driven via A.C. servomotors. Since no hydraulic control elements are used for regulation between the pumps and the hydraulic consumers, these systems work according to the principle of hydrostatic gearings. The speed control is effected by variation of the motor speed. By this measure a low-energy and noise-optimized operating method of the consumers can be achieved. The disadvantage in this device is the relatively expensive A.C. servomotors, which is, however, consciously accepted.